There are 4 types of gauges.
One uses a float, like a gasoline gauge on your car. It must be installed by a certified tank re-certifier It works very well and it is "Tank specific".
Most of the ones you find on the internet ared simple RED/Green pressure gauges. They do not tell you how much is left in the tank, only that you have LIQUID in the tank, can be 1 oz or 100 pounds. thy do not have the ability to tell the difference.
Next somes a scale. Those work based on the weight of cylinder and gas. These too are fairly accurate.
Finally there is a LCD strip.. YOu attach it to the tank (Either magnetic or peal and stick) Then put s pan of water on the stove to boil and pour the hot water over the strip. A marked color change will indicaste the liquid level in the tank.. Also rather accurate.